This disclosure relates to intervertebral disc prostheses, and more particularly to an intervertebral disc prosthesis that can be surgically introduced between adjacent vertebral bodies, preferably between lumbar vertebrae, and adjusted or expanded in situ to occupy an optimal or desired space between the vertebral bodies.
In recent years, surgical procedures have been developed in which two or more vertebrae are joined or fused together. Such procedures are now common in the treatment of spinal disorders such as spondylolisthesis, scoliosis, and disc degeneration. Certain of these fusion surgeries include Posterior Lumbar Interbody Fusion (PLIF), Transforaminal Lumbar Interbody Fusion (TLIF), Anterior Lumbar Interbody Fusion (ALIF), and DLIF (Direct Lateral Interbody Fusion). These procedures are well known to spinal surgeons.
Interbody vertebral spacers are known that are inserted between the vertebrae bodies to replace a collapsed, degenerated, or unstable disc. However, these prior spacers were typically of a predetermined thickness and thus it was difficult to size the thickness of the spacer to result in the desired amount of distraction between the vertebrae bodies in order to achieve the desired amount of distraction between the adjacent vertebra bodies. Such prior spacers are commercially available from Stryker Spine of Mahwah, N.J., from Medtronic, Spinal and Biologics Business, Memphis, Tenn., from Spinal Concepts, Inc. of Austin Tex., and from NuVasive, Inc. of San Diego, Calif.
Certain adjustable height interbody fusion devices are known, such as described in U.S. Pat. No. 6,080,193 that vary the distance between the portions of the spacer that engage the endplates of the adjacent vertebrae. However, these adjustable fusion devices rely on cams and other complicated mechanisms for adjustment purposes.
In general, lordosis is the curvature of the spine with the convexity forward. Lordosis is not necessarily a disease state, but rather the normal anterior physiologic curve of the neck or low back. This disclosure is primarily concerned with lumbar lordosis. Most lumbar disc spaces in healthy spines are generally parallel or nearly 0 degree lordotic, and is particularly true for L1/2, L2/3, and L 3/4 However, the L4/5 may have a lordotic angle ranging between about 0°-12°, and L5/S1 may also range between about 0°-12°. Therefore, in reconstructing a disc space that has some lordosis it would be advantageous to have an implant matching the existing anatomy so that the two surfaces of the implant would better conform to and better fit the shape of the disc space so that load sharing occurs over the whole implant. Otherwise, an implant having parallel upper and lower surfaces used in a disc space having, for example, 8 degrees of in situ lordosis would result in only part of the implant contacting its respective vertebrae bodies and thereby lessening the corrective support applied to the spine and thus predisposing the implant to subsidence. That is, subsidence refers to an increased tendency of the implant, over time, to telescope, settle or project into the adjacent vertebrae bodies with loss disc space height back to preoperative levels. When using conventional implants, it is frequently observed that such telescoping or settling occurs with such implants placed wholly within the disc space, rather than having the implant bearing on and distracting from the cortical rim/apophyseal ring of the adjacent vertebrae as disclosed in my co-pending U.S. patent application Ser. No. 12/899,625. Additionally, a more conformal fit in a disc space with lordosis will allow more uniform distribution of corrective forces applied during distraction. Thus, it would be desirable to provide an expandable (variable height) implant, as described in my above-noted pending application that would provide options for encountered or desired disc space lordosis. It would also be desirable to use either a variable or a fixed height implant that could reconstruct any disc space anatomy or morphology surgically encountered including lordotic conditions with the surgical procedures described in my above-identified U.S. patent applications and in the present disclosure.